This invention relates to a one-package system cold-setting type coating composition which comprises (i) an acrylic copolymer having carboxyl groups in its molecule or an alkyd resin, (ii) an aluminum alkoxide complex, (iii) a keto-enol tautomeric compound, (iv) a solvent and (v) an alkali metal hydroxide.
In general, acrylic coating compositions comprise an acrylic ester or a methacrylic ester as a main component and have excellent resistance to chemicals and weather and transparency. Their adhesive property, flexibility and hardness can be unrestrictedly controlled by copolymerizing other vinylic monomers therewith. Therefore, recently the use of acrylic coating compositions have increased more and more. Particularly, acrylic coatings with a three-dimensional structure obtained by curing the acrylic ester and/or the methacrylic ester have hard and tough properties and other excellent properties.
In general, coating compositions having hard and tough properties have been made by heating an acrylic resin and an amino resin, such as a urea resin, melamine resin or benzoguanamine resin at an elevated temperature.
Also, an acrylic resin which can give rise to three-dimensional cure at a room temperature as well as at a low temperature has been used in such particular use that a substrate to be coated or an environment of the coating step cannot be heated. In this case a so-called two packages system coating composition comprising an acrylic resin and a curing agent, such as isocyanate has generally been used. The method of carrying out cold-setting coating by using such a two-packages system coating composition, however, has the following disadvantages:
(a) Pot life after mixing a curing agent with a resin is short,
(b) Maintenance of the coating apparatus is complicated, and
(c) handling of the composition is inconvenient.
In order to overcome these disadvantages, research has been made as to acrylic coating compositions which are curable at a low temperature and which, when cured, can form a film with excellent resistance to chemicals, weather and solvent.
As a result, a one-package system cold-setting type acrylic coating composition having the excellent properties as mentioned above has been obtained by mixing a linear acrylic copolymer having carboxyl groups in its molecule, an aluminum alkoxide complex, a keto-enol tautomeric compound and a solvent as disclosed in U.S. Pat. No. 3,632,546 by Huang et al. which was assigned to the assignee of this application and which is incorporated herein.
Mechanism of a one-package system cold-setting type coating composition which comprises an acrylic copolymer having carboxyl groups in its molecule or an alkyd resin, an aluminum alkoxide complex, a keto-enol tautomeric compound and a solvent is not clear. It is believed that equilibrium between an aluminum alkoxide complex and a keto-enol tautomeric compound is formed in the composition.
That is, when mixing an aluminum alkoxide complex and a keto-enol tautomeric compound, it is believed that the following equilibrium is formed: EQU Al(OR).sub.2 (L.sub.1)+mL.sub.2 .revreaction.Al(OR).sub.2 --m(L.sub.1)(L.sub.2)m+mROH
wherein R is alkyl; L.sub.1 is keto-enol tautomeric compound, or a compound which can form complex with aluminum alkoxide, for example propanediol or derivative thereof, or a salicyloyl derivative, such as salicylic ester or salicyl aldehyde; L.sub.2 is keto-enol tautomeric compound; m is integer of 1 or 2. The coating composition is stable in the presence of the keto-enol tautomeric compound and the solvent, because the above equilibrium is maintained. However, when the coating composition is coated on the substrate, the keto-enol tautomeric compound and the solvent evaporate, whereby the composition gives rise to three-dimensional cure to form a hard and tough film having excellent properties.
Alkyd resin coating compositions comprise a polycondensate of a polyhydric alcohol and a polybasic acid as a main component. The properties and performances of the alkyd resin coating compositions can be varied widely by selecting the kinds of each of the polyhydric alcohol and polybasic acid or by adding a modifier, such as a fatty acid to the alkyd resin. In addition, the alkyd resin is low-priced. Therefore, recently a large amount of the alkyd resin has been used. Alkyd resins are classified into two large groups, thermo-setting type alkyd resin and cold-setting type alkyd resin. The former is classified into two groups according to the method for crosslinking, that is, into the method for heat-curing the alkyd resin through oxidation polymerization of the unsaturated fatty acid groups in the alkyd resin and the method for heat-curing the alkyd resin which comprises adding a urea resin or a melamine resin to the alkyd resin and allowing the crosslinking between the alkyd resin and the additive to occur. While the cold-setting type alkyd resin is cured by adding a large amount of drying oil, semi-drying oil or unsaturated fatty acids therefrom to the alkyd resin, followed by curing the mixture in the presence of a catalyst, such as compound of cobalt, manganese or lead at a room temperature. However, discoloration of coating film cannot be avoided in the coating composition containing the cold-setting type alkyd resin, because a large amount of drying oil, semi-drying oil or unsaturated fatty acids therefrom and a large amount of the metallic catalyst are present in the coating composition.
In order to avoid discoloration of the coating film caused by addition of a large amount of drying oil, semi-drying oil or unsaturated fatty acids and a large amount of the metallic catalyst, a one-package system cold-setting type coating composition may be prepared by incorporating the aluminum alkoxide compound and the keto-enol tautomeric compound as disclosed in U.S. Pat. No. 3,632,546 with the alkyd resin having a suitable acid value without using a large amount of drying oil, semi-drying oil or unsaturated fatty acids and a large amount of the metallic catalyst.
Nevertheless, when a one-package system cold-setting type acrylic coating composition or a one-package system cold-setting type alkyd resin coating composition containing the crosslinking agent as disclosed in U.S. Pat. No. 3,632,546 was coated on steel, particularly polished steel, it was found that discoloration of the coating film may occur. The undesirable discoloration of the coating film does not always occur. Whether or not the discoloration occurs depends on conditions under which the composition is coated, that is, the temperature and the humidity of coating environment, and the kinds of solvent employed. The higher the temperature or the humidity of the coating environment, the more striking the extent to which such discoloration occurs. Similarly, the thicker the coating of film is on the steel, the more striking the discoloration. Such discoloration occurs during the initial period of the coating step of the composition and the drying step.
In the specification, the term "discoloration of coating film" or "discoloration of coated substance" means the fact that undesirable color develops in the film or substance.
Two of the present inventors and other members who were all employees of the assignee of this application have found that such discoloration can be overcome by adding a nigrogen compound to the one-package system cold-setting type acrylic coating composition as disclosed in U.S. Pat. No. 3,632,546. Please refer to U.S. Pat. No. 4,058,497 patented on Nov. 15, 1977 which was assigned to the assignee of this application and which is incorporated herein. Though the addition of nitrogen compound to the composition as disclosed in U.S. Pat. No. 3,632,546 can prevent the discoloration of the coating film from occurring the physical properties of the coating film are lowered.